Lithium secondary battery including a negative electrode which is a sintered layer of silicon particles and/or silicon alloy particles and a nonaqueous electrolyte containing carbon dioxide dissolved therein and method for producing same

ABSTRACT

A rechargeable lithium battery including a negative electrode made by sintering, on a surface of a conductive metal foil as a current collector, a layer of a mixture of active material particles containing silicon and/or a silicon alloy and a binder, a positive electrode and a nonaqueous electrolyte, characterized in that the nonaqueous electrolyte contains carbon dioxide dissolved therein.

This application is a 371 of international applicationPCT/JP2004/007831, which claims priority based on Japanese patentapplication Nos. 2003-174672, 2003-298906, 2003-402902 and 2004-071483filed Jun. 19, 2003, Aug. 22, 2003, Dec. 2, 2003, and Mar. 12, 2004,respectively, which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a rechargeable lithium battery and alsoto a method for fabrication thereof.

BACKGROUND ART

As one of new types of high-power and high-energy density rechargeablebatteries, a rechargeable lithium battery has been recently utilizedwhich is charged and discharged by the transfer of lithium ions throughan on aqueous electrolytes olution between the positive and negativeelectrodes.

For such a rechargeable lithium battery, a negative electrode using alithium-alloying material, such as silicon, for the negative activematerial has been studied. However, in the case where thelithium-alloying material, such as silicon, is used as the activematerial of the negative electrode, the active material is powdered ordelaminated from the current collector during charge and dischargebecause the active material expands and shrinks in volume when it storesand releases lithium. This lowers a current-collecting capacity of theelectrode and accordingly deteriorates charge-discharge cycleperformance characteristics, which has been a problem.

In order to solve the above-described problem, the present applicant hasproposed a negative electrode, for use in rechargeable lithiumbatteries, which is obtained by providing, on a surface of a currentcollector, a layer of a mixture containing a binder and active materialparticles containing silicon and/or a silicon alloy and then sinteringthe mixture layer while placed on the current collector (Patent Document1).

For rechargeable lithium batteries using carbon material or metalliclithium as a negative active material, dissolving carbon dioxide innonaqueous electrolytes or encapsulating carbon dioxide in a battery hasbeen proposed (Patent Documents 2-12).

The rechargeable lithium battery described above as a proposal of thepresent applicant exhibits a high charge-discharge capacity and showssuperior cycle performance characteristics. However, the active materialparticles in the negative electrode increase in porosity with repetitivecharge-discharge cycling to result in the increased thickness of thenegative electrode, which has been a problem.

-   Patent Document 1: PCT Int. Publication No. WO 02/21,616-   Patent Document 2: U.S. Pat. No. 4,853,304-   Patent Document 3: Japanese Patent Laid-Open No. Hei 6-150975-   Patent Document 4: Japanese Patent Laid-Open No. Hei 6-124700-   Patent Document 5: Japanese Patent Laid-Open No. Hei 7-176323-   Patent Document 6: Japanese Patent Laid-Open No. Hei 7-249431-   Patent Document 7: Japanese Patent Laid-Open No. Hei 8-64246-   Patent Document 8: Japanese Patent Laid-Open No. Hei 9-63649-   Patent Document 9: Japanese Patent Laid-Open No. Hei 10-40958-   Patent Document 10: Japanese Patent Laid-Open No. 2001-307771-   Patent Document 11: Japanese Patent Laid-Open No. 2002-329502-   Patent Document 12: Japanese Patent Laid-Open No. 2003-86243

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a rechargeablelithium battery using a negative electrode including active materialparticles containing silicon and/or a silicon alloy, which has a highcharge-discharge capacity and shows superior cycle performancecharacteristics and which can suppress increase in porosity of theactive material particles during charge and discharge and accordinglyreduce an increase in thickness of the electrode after charge anddischarge, as well as providing a method for fabrication thereof.

The rechargeable lithium battery of the present invention includes anegative electrode made by sintering a layer of a mixture of activematerial particles containing silicon and/or a silicon alloy and abinder on a surface of a conductive metal foil current collector, apositive electrode and a nonaqueous electrolyte. Characteristically, thenonaqueous electrolyte contains carbon dioxide dissolved therein.

In the present invention, the nonaqueous electrolyte contains carbondioxide dissolved therein. This means that the nonaqueous electrolytecontains carbon dioxide purposely or intentionally dissolved therein.Although carbon dioxide inevitably dissolves in a nonaqueous electrolyteduring a general fabrication process of rechargeable lithium batteries,carbon dioxide such dissolved is not meant to be included within thescope. Carbon dioxide generally dissolves in a solvent of a nonaqueouselectrolyte. Thus, the nonaqueous electrolyte may be prepared bydissolving a solute and then carbon dioxide into a solvent.Alternatively, the nonaqueous electrolyte may be prepared by dissolvingcarbon dioxide and then a solute into a solvent.

A porosity increase of the active material particles, which occurs witha charge-discharge reaction, can be retarded by dissolving carbondioxide in a nonaqueous electrolyte. Accordingly, a thickness increaseof a layer of active material particles during charge and discharge canbe suppressed to result in the increased volumetric energy density ofthe rechargeable lithium battery.

The negative electrode prepared by sintering a layer of a mixture ofactive material particles containing silicon and/or a silicon alloy anda binder on a surface of a conductive metal foil current collectorexhibits a high charge-discharge capacity and shows superiorcharge-discharge performance characteristics. The inventors of thisapplication have found that, as a charge-discharge reaction is repeatedin such an electrode, the active material particle shows a gradualporosity increase that starts from its surface and develops toward itsinside. As the porosity increases, the thickness of the electrodeincreases. As a result, the volumetric energy density of the electrodedecreases. Such porosity increase of the active material is believed dueto the property change of the silicon active material that occurs as itundergoes an irreversible reaction.

Dissolving of carbon dioxide in a nonaqueous electrolyte, in accordancewith the present invention, suppresses a porosity increase of the activematerial. This accordingly suppresses a thickness increase of theelectrode and thereby increases a volumetric energy density of theelectrode. The detailed reason why the porosity increase of the activematerial particle can be suppressed when a nonaqueous electrolytecontains an amount of dissolved carbon dioxide is not clear, but is mostprobably due to the formation of a stable film having a superiorlithium-ion conducting capability on a particle surface.

In the present invention, in the preparation of the negative electrode,sintering is preferably performed under a non-oxidizing atmosphere.

In the present invention, the amount of carbon dioxide dissolved in anonaqueous electrolyte is preferably at least 0.001% by weight, morepreferably at least 0.01% by weight, further preferably at least 0.05%by weight, further preferably at least 0.1% by weight. It is generallypreferred that carbon dioxide is dissolved in a nonaqueous electrolyteto saturation. The above-specified amount of dissolved carbon dioxidedoes not include the amount of carbon dioxide which inevitably dissolvesin a nonaqueous electrolyte, i.e., excludes the amount of carbon dioxidewhich dissolves in a nonaqueous electrolyte during a general fabricationprocess of rechargeable lithium batteries. The above-specified amount ofdissolved carbon dioxide can be determined by measuring a weight of anonaqueous electrolyte both after and before dissolving of carbondioxide in the nonaqueous electrolyte. Specifically, the amount ofdissolved carbon dioxide can be calculated using the following equation:

Amount of carbon dioxide dissolved in a nonaqueous electrolyte (weight%)=[(weight of the nonaqueous electrolyte after dissolving of carbondioxide therein)−(weight of the nonaqueous electrolyte before dissolvingof carbon dioxide therein)]/(weight of the nonaqueous electrolyte afterdissolving of carbon dioxide therein)×100.

In the present invention, it is preferred that carbon dioxide is alsocontained in an inner space of the battery. Such an inner space of thebattery may be provided between a battery casing and an electrodeassembly which includes opposing positive and negative electrodes and aseparator sandwiched between them, for example. Carbon dioxide can becontained in the space by performing battery assembling under a carbondioxide atmosphere or by allowing release of the dissolved carbondioxide from the electrolyte into the space. As carbon dioxide in theelectrolyte is consumed during charge and discharge, the carbon dioxidein the space dissolves into the electrolyte so that carbon dioxide canbe supplied into the electrolyte.

In this invention, the nonaqueous electrolyte preferably contains afluorine-containing compound. Inclusion of such a compound in thenonaqueous electrolyte further improves cycle performancecharacteristics.

Examples of fluorine-containing compounds include fluorine-containinglithium salts and fluorine-containing solvents.

Examples of such fluorine-containing lithium salts include LiPF₆, LiBF₄,LiCF₃SO₃, LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, LiN(CF₃SO₂)(C₄F₉SO₂),LiC(CF₃SO₂)₃, LiC(C₂F₅SO₂)₃, LiAsF₆, LiXF_(y) (wherein X is P, As, Sb,B, Bi, Al, Ga or In; y is 6 if X is P, As or Sb and y is 4 if X is B,Bi, Al, Ga or In), lithium perfluoroalkylsulfonyl imideLiN(C_(m)F_(2m+1)SO₂)(C_(n)F_(2n+1)SO₂) (wherein m and n areindependently integers of 1-4), lithium perfluoroalkylsulfonyl methideLiC(C_(p)F_(2p+1)SO₂)(C_(q)F_(2q+1)SO₂)(C_(r)F_(2r+1)SO₂) (wherein p, qand r are independently integers of 1-4), and the like.

Examples of fluorine-containing solvents include compounds derived bysubstituting fluorine atoms for hydrogen atoms in cyclic carbonates,such as butylene carbonate and propylene carbonate, and in chaincarbonates such as dimethyl carbonate and diethyl carbonate. Specificexamples include trifluoromethylated propylene which is derived bysubstituting fluorine atoms for hydrogen atoms in propylene carbonate,1,1,1-trifluorodiethyl carbonate (CF₃CH₂OCOOCH₂CH₃), trifluoro ethylmethyl carbonate (CF₃CH₂OCOOCH₃). Other useful compounds include thosederived by substituting fluorine atoms for hydrogen atoms in ethersolvents, such as 1,2-dimethoxyethane and 1,2-diethoxyethane, and incyclic esters such as γ-butyrolactone. A specific example isbis-1,2-(2,2,2-trifluoroethoxy) ethane (CF₃CH₂OCH₂CH₂OCH₂CF₃).

In the case where the fluorine-containing lithium salt is used as asolute for the nonaqueous electrolyte, it is added preferably in theconcentration of 0.1-2 mole/liter of the nonaqueous electrolyte. A totalamount of the lithium salt is preferably 0.5-2 mole/liter. If theconcentration is below 0.1 mole/liter, the effect of containing fluorinemay not be obtained sufficiently. If the total amount of the lithiumsalt is below 0.5 mole/liter, a sufficient lithium-ion conductingcapability may not be obtained for the nonaqueous electrolyte. If theconcentration exceeds 2 mole/liter, the nonaqueous electrolyte mayundesirably increase in viscosity and decrease in ionic conductivity.Also, a salt may be undesirably separated out at low temperatures.

Where the fluorine-containing compound is used as a solvent for thenonaqueous electrolyte, it is preferably used in the concentration of atleast 1% of the total volume of all solvents. If the concentration isbelow 1% by volume, the effect of containing fluorine may not beobtained sufficiently.

In the present invention, a fluorine-containing compound of the typethat is hard to dissolve in the electrolyte may be preloaded in theseparator. Also, a fluorine-containing compound may be preloaded in theanode mix layer. Such a compound can be illustrated by lithium fluoride.

Where the fluorine-containing compound is added to the anode mix layer,it is preferably loaded in the amount of 0.05-5% of the total weight ofthe anode mix. If below 0.05% by weight, the effect of containingfluorine may not be obtained sufficiently. On the other hand, if above5% by weight, a resistance of the active material layer may increase toan undesirable level.

The active material particles for use in the present invention may becomposed of silicon and/or a silicon alloy. Examples of silicon alloysinclude solid solutions of silicon and other one or more elements,intermetallic compounds of silicon with other one or more elements andeutectic alloys of silicon and other one or more elements. Alloying canbe achieved by such methods as arc melting, liquid quenching, mechanicalalloying, sputtering, chemical vapor growth and firing. Examples ofliquid quenching methods include a single roller quenching method, atwin roller quenching method and various atomizing methods including gasatomizing, water atomizing and disk atomizing.

The active material particles for use in the present invention may alsocomprise silicon and/or silicon alloy particles with surfaces beingcoated with a metal or the other. Coating can be achieved by suchmethods as electroless plating, electrolytic plating, chemicalreduction, vapor deposition, sputtering and chemical vapor deposition.Preferably, the coating metal is the same type of metal as the metalfoil current collector. In the sintering, the active material particlesif coated with the metal identical in type to the metal foil exhibit amarked improvement in adhesion to the current collector. As a result,further improved charge-discharge cycle performance characteristics canbe obtained.

The active material particles for use in the present invention mayinclude particles composed of material that alloys with lithium.Examples of lithium-alloying materials include germanium, tin, lead,zinc, magnesium, sodium, aluminum, gallium, indium and their alloys.

A mean particle diameter of the active material particles for use in thepresent invention is not particularly specified but may preferably be100 μm or below, more preferably 50 μm or below, most preferably 10 μmor below, to insure effective sintering. The better cycle performancecharacteristics can be obtained as the mean particle diameter of theactive material particles becomes smaller. A mean particle diameter ofthe conductive powder for incorporation in the mix layer is notparticularly specified but may preferably be up to 100 μm, morepreferably up to 50 μm, most preferably up to 10 μm.

The use of active material particles having a smaller mean particlediameter reduces an absolute amount of volumetric expansion andshrinkage of the active material as it stores and releases lithium by acharge-discharge reaction and accordingly reduces an absolute amount ofa strain that is produced between active material particles in theelectrode during a charge-discharge reaction. This prevents breakage ofthe binder, suppresses reduction of a current-collecting capability ofthe electrode and improves charge-discharge cycle performancecharacteristics.

It appears in the present invention that carbon dioxide dissolved in thenonaqueous electrolyte acts to form a stable film having a highlithium-ion conducting capability on a surface of the active materialparticle, as described earlier. The use of active material particleshaving a smaller mean particle diameter then results in the denserarrangement of the high lithium-ion conducting films throughout the mixlayer. The formation of the denser lithium-ion conducting paths in themix layer is believed to allow a charge-discharge reaction to occur inmore uniformly distributed regions in the electrode. This prevents theactive material from being broken by a strain produced when the activematerial undergoes a biased volumetric change as it stores and releaseslithium, and accordingly reduces the tendency of the active materialparticles to form new surfaces and, as a result, further improvescharge-discharge cycle performance characteristics.

The active material particles preferably have as narrow a sizedistribution as possible. The wide particle size distribution creates alarge difference between the active material particles having largelydiffering sizes, in terms of an absolute amount of volumetric expansionor shrinkage of the active material particle as it stores and releaseslithium. This large difference produces a strain in the anode mix layerthat causes breakage of the binder. The current-collecting capability ofthe electrode then decreases to thereby deteriorate cycle performancecharacteristics.

A surface of the current collector in the present invention preferablyhas an arithmetic mean roughness Ra of at least 0.2 μm. The use of thecurrent collector having the above-specified arithmetic mean surfaceroughness Ra increases a contact area of the current collector with themix layer and accordingly improves adhesion between them. This furtherimproves a current-collecting capability of the electrode. In the casewhere the mix layer is disposed each surface of the current collector,the current collector preferably has an arithmetic mean surfaceroughness Ra of at least 0.2 μm.

The arithmetic mean roughness Ra is defined in Japanese IndustrialStandards (JIS B 0601-1994) and can be measured as by a surfaceroughness meter.

In the present invention, the thickness of the current collector is notparticularly specified, but is preferably in the range of 10-100 μm.

In the present invention, an upper limit of the arithmetic meanroughness Ra of the current collector surface is not particularlyspecified. However, its substantial value is preferably 10 μm or belowbecause the thickness of the current collector is preferably in therange of 10-100 μm, as described above.

The current collector in the present invention preferably comprises anelectrically conductive metal foil which may be composed of a metal suchas copper, nickel, iron, titanium or cobalt, or an alloy containing anycombination thereof, for example. It is particularly preferred that theconductive metal foil contains a metal element that readily diffusesinto the active material. From this point of view, the conductive metalfoil preferably comprises a copper foil or a copper alloy foil. Sincecopper, when heat treated, readily diffuses into the silicon activematerial, sintering is expected to improve adhesion between the currentcollector and the active material. For this purpose, the currentcollector may comprise a metal foil having a layer containing a copperelement on its surface in contact with the active material. Therefore,in the case where a metal foil composed of a metal element other thancopper is used, a copper or copper alloy layer is preferably provided ona surface of the metal foil.

The preferred copper alloy foil is a heat-resisting copper foil. Theheat-resisting copper alloy, as used herein, refers to a copper alloywhich exhibits a tensile strength of at least 300 MPa after one hour ofannealing at 200° C. Examples of useful heat-resisting copper alloys arelisted in Table 1.

TABLE 1 (% on a Weight Basis) Type Composition Tin-Containing 0.05-0.2%Sn and 0.04% or Less P Added to Cu Copper Silver-Containing 0.08-0.25%Ag Added to Cu Copper Zirconium-Copper 0.02-0.2% Zr Added to Cu (Used inExamples) Chromium-Copper 0.4-1.2% Cr Added to Cu Titanium-Copper1.0-4.0% Ti Added to Cu Beryllium-Copper 0.4-2.2% Be, Slight Amounts ofCo, Ni and Fe Added to Cu Iron-Containing 0.1-2.6% Fe and 0.01-0.3% PAdded to Cu Copper High-Strength Brass 2.0% or Less Al, 3.0% or Less Mnand 1.5% or Less Fe Added to Brass of 55.0-60.5% Cu Tin-Containing Brass80.0-95.0% Cu, 1.5-3.5% Sn and a Balance of Zn Phosphor Bronze Mainly ofCu and Containing 3.5-9.0% Sn and 0.03-0.35% P Aluminum BronzeContaining 77.0-92.5% Cu, 6.0-12.0% Al, 1.5-6.0% Fe, 7.0% or Less Ni and2.0% or Less Mn White Copper Mainly of Cu and Containing 9.0-33.0% Ni,0.40-2.3% Fe, 0.20-2.5% Mn and 1.0% or Less Zn Corson Alloy 3% Ni, 0.65%Si and 0.15% Mg in Cu Cr—Zr—Cu Alloy 0.2% Cr, 0.1% Zr and 0.2% Zn in Cu

As described earlier, the current collector for use in the presentinvention preferably has large irregularities on its surface. Unless thearithmetic mean roughness Ra of the heat-resisting copper alloy foil issufficiently large, an electrolytic copper or copper alloy may besuperimposed on a surface of the foil to provide large irregularities onthe surface. Such electrolytic copper and copper alloy layers can beformed through an electrolytic process.

Also in the present invention, the current collector may be subjected toa surface-roughening treatment to provide large irregularities on itssurface. Examples of such surface-roughening treatments include vaporgrowth processes, etching and polishing. Examples of vapor growthprocesses include sputtering, CVD and vapor deposition. Etching may beachieved either physically or chemically. Polishing may be carried outusing a sand paper or with blast.

In the present invention, a thickness X of the mix layer, a thickness Yof the current collector and an arithmetic mean roughness Ra of itssurface preferably meet the relationships 5Y≧X and 250Ra≧X. If thethickness X of the mix layer exceeds 5Y or 250Ra, the occasionalseparation of the mix layer from the current collector may result.

The thickness X of the anode mix layer is not particularly specified butis preferably 1,000 μm or below, more preferably 10 μm-100 μm.

In the present invention, an electrically conductive powder can beincorporated in the mix layer. Such a conductive powder, when loaded,surrounds particles of active material to form an electricallyconductive network, resulting in further improving thecurrent-collecting capability of the electrode. The conductive powder ispreferably made from the same material as contained in the currentcollector. Specific examples of useful materials include metals such ascopper, nickel, iron, titanium and cobalt; alloys and mixtures of anycombination thereof. Among those metal powders, a copper powder isparticularly useful. The use of a conductive carbon powder is alsopreferred.

Preferably, the conductive powder is loaded in the mix layer in theamount that does not exceed 50% of the total weight of the conductivepowder and active material particles. If the loading of the conductivepowder is excessively high, an amount of the active material particlesbecomes relatively small to result in reducing a charge-dischargecapacity of the electrode.

The binder for use in the present invention is preferably of the typethat remains fully undecomposed after the heat treatment for sintering.As stated above, sintering improves adhesion between the active materialparticles and the current collector and between the active materialparticles themselves. If the binder remains undecomposed even after theheat treatment, the binding ability thereof further improves suchadhesion. Also, the use of a metal foil having an arithmetic meansurface roughness Ra of at least 0.2 μm as the current collector allowsthe binder to penetrate into recesses on a surface of the currentcollector. Then, an anchor effect is created between the binder and thecurrent collector to further improve their adhesion. Accordingly, evenif the active material expands and shrinks in volume as lithium isstored and released, shedding of the active material from the currentcollector can be restrained to result in obtaining satisfactorycharge-discharge cycle performance characteristics.

The binder for use in the present invention preferably comprisespolyimide. Polyimide, either thermoplastic or thermosetting, are useful.Also, polyimide can be obtained, for example, by subjecting polyamicacid to a heat treatment.

The heat treatment causes polyamic acid to undergo dehydrocondensationto produce polyimide. Preferably, such polyimide has an imidizationlevel of at least 80%. The imidization level, as used herein, refers toa mole % of the produced polyimide relative to a polyimide precursor(polyamic acid). Polyimide with at least 80% imidization level can beobtained, for example, by subjecting an N-methyl-2-pyrrolidone (NMP)solution of polyamic acid to a heat treatment at a temperature of 100°C.-400° C. for over 1 hour. In an exemplary case where the heattreatment is carried out at 350° C., the imidization level reaches 80%in about 1 hour and 100% in about 3 hours.

In case of using polyimide as a binder, sintering is preferably carriedout at a temperature insufficient to cause decomposition of polyimide,i.e., at 600° C. or below, because the binder in the present inventionis preferred to remain fully undecomposed even after the heat treatmentfor sintering.

In the present invention, the amount by weight of the binder in the mixlayer is preferably at least 5% of the total weight of the mix layer.Also preferably, the binder volume is at least 5% of the total volume ofthe mix layer. If the amount of the binder in the mix layer isexcessively small, adhesion between the components within the electrodemay become insufficient. If the amount of the binder in the mix layer isexcessively large, an internal resistance of the electrode increases tooccasionally result in the difficulty to initiate a charge. Accordingly,it is preferred that the amount by weight of the binder in the mix layerdoes not exceed 50% of the total weight of the mix layer. It is alsopreferred that the binder volume does not exceed 50% of the total volumeof the mix layer.

The rechargeable lithium battery, according to another aspect of thepresent invention, includes a negative electrode prepared by providing,on a surface of a conductive metal foil as a current collector, a mixlayer containing a binder and active material particles having atendency to undergo a porosity increase that advances inside fromparticle surfaces during charge and discharge, and also includes apositive electrode and a nonaqueous electrolyte. Characteristically, thenonaqueous electrolyte contains carbon dioxide dissolved therein.

The active material particles which undergo a porosity increase thatadvances inside from particle surfaces during charge and discharge canbe illustrated by silicon particles and silicon alloy particles. Bydissolving carbon dioxide in the nonaqueous electrolyte, the occurrenceof porosity increase of the active material particles and thicknessincrease of the electrode during charge and discharge is effectivelyreduced to result in the increased volumetric energy density of thebattery.

A solvent of the nonaqueous electrolyte for use in the rechargeablelithium battery of the present invention is not particularly specifiedin type but can be illustrated by cyclic and chain carbonates. Examplesof cyclic carbonates include ethylene carbonate, propylene carbonate,butylene carbonate and vinylene carbonate. Examples of chain carbonatesinclude dimethyl carbonate, methyl ethyl carbonate and diethylcarbonate. The presence of the cyclic carbonate in the solvent of thenonaqueous electrolyte is particularly beneficial to formation of a filmhaving a superior lithium-ion conducting capability on surfaces ofactive material particles. The use of the cyclic carbonate is thereforepreferred. Particularly preferred are ethylene carbonate and propylenecarbonate. A mixed solvent of a cyclic carbonate and a chain carbonateis also preferably used. It is particularly preferred that such a mixedsolvent contains ethylene carbonate or propylene carbonate, and diethylcarbonate.

Also applicable is a mixed solvent which contains any of theabove-listed cyclic carbonates and, an ether solvent such as1,2-dimethoxyethane or 1,2-diethoxyethane or a chain ester such asγ-butyrolactone, sulfolane or methyl acetate.

Also, a solute of the nonaqueous electrolyte can be illustrated byLiPF₆, LiBF₄, LiCF₃SO₃, LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂,LiN(CF₃SO₂)(C₄F₉SO₂), LiC(CF₃SO₂)₃, LiC(C₂F₅SO₂)₃, LiAsF₆, LiClO₄,Li₂B₁₀Cl₁₀, Li₂B₁₂Cl₁₂ and mixtures thereof. Particularly preferred isthe use of a mixed solute of LiXF_(y) (wherein X is P, As, Sb, B, Bi,Al, Ga or In; y is 6 if X is P, As or Sb and 4 if X is B, Bi, Al, Ga orIn) with lithium perfluoroalkylsulfonyl imideLiN(C_(m)F_(2m+1)SO₂)(C_(n)F_(2n+1)SO₂) (wherein m and n areindependently integers of 1-4) or with lithium perfluoroalkylsulfonylmethide LiN(C_(p)F_(2p+1)SO₂)(C_(q)F_(2q+1)SO₂)(C_(r)F_(2r+1)SO₂)(wherein p, q and r are independently integers of 1-4). Among them, theuse of LiPF₆ is particularly preferred.

Applicable electrolytes include, for example, gelled polymerelectrolytes comprised of an electrolyte solution impregnated intopolymer electrolytes such as polyethylene oxide and polyacrylonitrile;and inorganic solid electrolytes such as LiI and Li₃N. The electrolytefor the rechargeable lithium battery of the present invention can beused without limitation, so long as a lithium compound as its solutethat imparts ionic conductivity, as well as its solvent that dissolvesand retains the lithium compound, remain undecomposed at voltages duringcharge, discharge and storage of the battery.

Examples of useful positive electrode materials for the rechargeablelithium battery of the present invention include lithium-containingtransition metal oxides such as LiCoO₂, LiNiO₂, LiMn₂O₄, LiMnO₂,LiCO_(0.5)Ni_(0.5)O₂ and LiNi_(0.7)CO_(0.2)Mn_(0.1)O₂; and lithium-freemetal oxides such as MnO₂. Other substances can also be used, withoutlimitation, if they are capable of electrochemical lithium insertion anddeinsertion.

The method of the present invention enables fabrication of theabove-described rechargeable lithium battery of the present inventionand is characterized as including the steps of providing a layer of amixture of active material particles containing silicon and/or a siliconalloy and a binder on a surface of a conductive metal foil as a currentcollector and then sintering the mixture layer while being placed on theconductive metal foil to prepare a negative electrode, dissolving carbondioxide in a nonaqueous electrolyte, and assembling a rechargeablelithium battery using the negative electrode, a positive electrode andthe nonaqueous electrode.

In the fabrication method of the present invention, sintering ispreferably performed under a non-oxidizing atmosphere to provide thenegative electrode.

Various methods can be utilized to dissolve carbon dioxide in thenonaqueous electrolyte. For example, carbon dioxide is forced to contactwith the nonaqueous electrolyte. Specifically, a carbon dioxide gas isblown into the nonaqueous electrolyte. This is an efficient and easymethod resulting in obtaining the nonaqueous electrolyte containingdissolved carbon dioxide. Other useful methods include stirring thenonaqueous electrolyte under a carbon dioxide atmosphere, and contactinga high-pressure stream of carbon dioxide with the nonaqueouselectrolyte. Alternatively, a carbon dioxide generator may be added todissolve carbon dioxide in the nonaqueous electrolyte. Examples ofcarbon dioxide generators include polycarbonates and carbonates. Dry icemay also be used.

In the fabrication of a rechargeable lithium battery using thenonaqueous electrolyte containing dissolved carbon dioxide, it ispreferred that the amount of carbon dioxide dissolved in the nonaqueouselectrolyte is stably controlled. To this object, a rechargeable lithiumbattery is preferably assembled under the atmosphere including carbondioxide. For example, a step of introducing the nonaqueous electrolytecontaining dissolved carbon dioxide into the battery and the subsequentsteps are preferably performed under the atmosphere including carbondioxide. It is also preferred that, after being introduced into thebattery, the nonaqueous electrolyte containing dissolved carbon dioxideis exposed to a high-pressure carbon dioxide atmosphere to stabilize theamount of dissolved carbon dioxide. The amount of carbon dioxide thatcan be dissolved to saturation varies with a temperature of thenonaqueous electrolyte. It is accordingly preferred that, in thefabrication steps, a control is provided to minimize a temperaturevariation of the rechargeable lithium battery.

Fabrication of the rechargeable lithium battery of the present inventionmay be carried out under the atmosphere including carbon dioxide so thatcarbon dioxide is dissolved in the nonaqueous electrolyte. For example,the battery before being sealed is left to stand under the atmosphereincluding carbon dioxide and is then sealed after a predetermined periodof time to thereby dissolve carbon oxide in the nonaqueous electrolyte.

In the present invention, the mixture layer can be placed on a surfaceof the metal foil current collector by dispersing the active materialparticles in a binder solution to provide a slurry and then applying theslurry onto the surface of the metal foil current collector.

In the fabrication of the present invention, subsequent to provision ofthe mixture layer on a surface of the metal foil current collector butprior to sintering, the mixture layer and the underlying metal foilcurrent collector are preferably rolled together. Such rolling increasesa packing density of the mixture layer and thus improves adhesionbetween active material particles and between the mixture layer and thecurrent collector, resulting in obtaining further improvedcharge-discharge cycle performance characteristics.

In the present invention, sintering is preferably carried out under anon-oxidizing atmosphere such as a vacuum atmosphere, or a nitrogen,argon or other inert gas atmosphere. Sintering may also be carried outunder a hydrogen or other reducing atmosphere. Preferably, sintering isaccomplished by a heat treatment at a temperature that does not exceedmelting points of the metal foil current collector and the activematerial particles. For example, when a copper foil is used as the metalfoil current collector, the heat treatment temperature is preferablycontrolled not to exceed a melting point of copper, i.e., 1083° C. It ismore preferably within the range of 200-500° C., further preferablywithin the range of 300-450° C. Sintering can be achieved by a sparkplasma sintering or hot pressing technique.

In accordance with the present invention, a rechargeable lithium batterycan be provided which shows a high charge-discharge capacity andsuperior cycle characteristics, and in which a porosity increase of theactive material particles during charge and discharge can be suppressedand accordingly a thickness increase of the electrode after charge anddischarge can be retarded.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an FIB-SIM image showing a section of the negative electrodeof the rechargeable lithium battery A1 in accordance with the presentinvention;

FIG. 2 is an FIB-SIM image showing a section of the negative electrodeof the rechargeable lithium battery A1 in accordance with the presentinvention;

FIG. 3 is an FIB-SIM image showing a section of the negative electrodeof the comparative battery B1;

FIG. 4 is an FIB-SIM image showing a section of the negative electrodeof the comparative battery B1;

FIG. 5 is a spectrum showing the TOF-SIMS surface analysis results(positive ions) for the negative electrode;

FIG. 6 is a spectrum showing the TOF-SIMS surface analysis results(negative ions) for the negative electrode;

FIG. 7 is a plan view, showing the rechargeable lithium batteryfabricated in the Example in accordance with the present invention;

FIG. 8 is a sectional view, showing a section of the rechargeablelithium battery shown in FIG. 7; and

FIG. 9 is a graph showing the relationship between the cycle life andthe amount of carbon dioxide dissolved in the nonaqueous electrolyte.

EXPLANATION OF THE REFERENCE NUMERALS

1 outer casing 2 sealed portion 3 positive current-collecting tab 4negative current-collecting tab 5 electrode assembly

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is below described in more detail by way ofExamples. The following examples merely illustrate the practice of thepresent invention but are not intended to be limiting thereof. Suitablechanges and modifications can be effected without departing from thescope of the present invention.

EXPERIMENT 1

(Preparation of Negative Electrode)

81.8 parts by weight of silicon powder (99.9% pure) having a meanparticle diameter of 3 μm as the active material particles was mixed in8.6 wt. % N-methylpyrrolidone solution containing 18.2 parts by weightof polyimide as a binder to provide an anode mix slurry.

This anode mix slurry was coated on one surface (rough surface) of anelectrolytic copper foil (35 μm thick) (current collector a1) having anarithmetic mean surface roughness Ra of 0.5 μm and then dried. A 25mm×30 mm rectangular piece was cut out from the coated copper foil,rolled and then sintered by a heat treatment under argon atmosphere at400° C. for 30 hours to provide a negative electrode. The sintered body(inclusive of the current collector) was 50 μm thick. Accordingly, thethickness of the anode mix layer was 15 μm, anode mix layerthickness/copper foil arithmetic mean surface roughness was 30, andanode mix layer thickness/copper foil thickness was 0.43.

In the negative electrode, polyimide was found to have a density of 1.1g/cm³ and constitute 31.8% of the total volume of the anode mix layer.

(Preparation of Positive Electrode)

Starting materials, Li₂CO₃ and CoCO₃, were weighed such that a ratio ofnumbers of Li and Co atoms, Li:Co, was brought to 1:1, and then mixed ina mortar. The mixture was pressed in a 17 mm diameter mold and fired inthe air at 800° C. for 24 hours to obtain a fired product consisting ofLiCoO₂. This product was then ground into particles with a mean particlediameter of 20 μm.

90 parts by weight of the resulting LiCoO₂ powder and 5 parts by weightof artificial graphite as an electric conductor were mixed in a 5 wt. %N-methylpyrrolidone (NMP) solution containing 5 parts by weight ofpolyvinylidene fluoride as a binder to provide a cathode mix slurry.

The cathode mix slurry was coated on an aluminum foil as a currentcollector, dried and then rolled. A 20 mm×20 mm square piece was cut outfrom the coated aluminum foil to provide a positive electrode.

(Preparation of Electrolyte Solution)

1 mole/liter of LiPF₆ was dissolved in a mixed solvent containingethylene carbonate and diethyl carbonate at a 3:7 ratio by volume toprepare an electrolyte solution x. This electrolyte solution x wascooled to 5° C. Thereafter, a carbon dioxide gas was blown at a flowrate of 300 ml/min into the electrolyte solution x under a carbondioxide atmosphere for about 30 minutes until a weight change of theelectrolyte solution was leveled off. The resultant was elevated intemperature to 25° C. to prepare an electrolyte solution a1.

A weight of the electrolyte subsequent to and prior to blowing of acarbon oxide gas was measured and the amount of carbon dioxide dissolvedin the electrolyte solution a1 was determined to be 0.37 weight %. Theweight of the electrolyte solution subsequent to blowing of a carbondioxide gas was measured under a carbon dioxide gas atmosphere.

(Fabrication of Battery)

The thus-prepared positive electrode, negative electrode and electrolytesolution were inserted into an outer casing made of an aluminum laminateto fabricate a rechargeable lithium battery A1. Fabrication of thisrechargeable lithium battery was performed under a carbon dioxide gasatmosphere at ambient temperature and pressure.

FIG. 7 is a front view showing the rechargeable lithium batteryfabricated. FIG. 8 is a sectional view taken along the line A-A of FIG.7. The positive and negative electrodes are placed on opposite sides ofa porous polyethylene separator to constitute an electrode assembly 5for insertion into the outer casing 1 made of an aluminum laminate, asshown in FIG. 8. A positive current collecting tab 3, made of aluminum,is attached to the positive electrode such that its leading end extendsthrough the outer casing 1 to an outside. A negative current collectingtab 4, made of nickel, is attached to the negative electrode such thatits leading end extends through the outer casing 1 to an outside. Asshown in FIGS. 7 and 8, the outer casing 1 is welded at its periphery todefine a sealed portion 2.

EXPERIMENT 2

Carbon dioxide was not blown into the electrolyte solution x inExperiment 1. Instead, 5 weight % of vinylene carbonate was addedthereto to prepare an electrolyte solution b1. The procedure ofExperiment 1 was followed, except that the above-prepared electrolytesolution b1 was used and battery fabrication was performed under argonatmosphere, to fabricate a battery B1.

(Evaluation of Charge-Discharge Cycle Characteristics)

The above-fabricated batteries A1 and B1 were evaluated forcharge-discharge cycle performance characteristics. Each battery wascharged at 25° C. at a constant current of 14 mA to 4.2 V, charged at aconstant voltage of 4.2 V to a current of 0.7 mA and then discharged ata current of 14 mA to 2.75 V. This was recorded as a unit cycle ofcharge and discharge. The battery was cycled to determine the number ofcycles after which its discharge capacity fell down to 80% of itsfirst-cycle discharge capacity and the determined cycle number wasrecorded as a cycle life. The results are shown in Table 2. The cyclelife of each battery is indicated therein by an index when that of thebattery A1 is taken as 100.

TABLE 2 Amount of Carbon Dioxide Dissolved in Electrolyte SolutionBattery (wt. %) Cycle Life A1 0.37 100 B1 0 48

As can be clearly seen from Table 1, the battery A1 using theelectrolyte solution a1 containing dissolved carbon dioxide shows alonger cycle life, compared to the battery B1 using the electrolytesolution b1 without dissolved carbon dioxide. The reason why vinylenecarbonate is added to the electrolyte solution b1 is that withoutaddition of carbon dioxide and vinylene carbonate to the electrolytesolution, the cycle life may become very short to result in thedifficulty to observe porosity increase of the active material.

(FIB-SIM Observation)

After the above-described charge-discharge cycle test, the batteries A1and B1 were disassembled to remove respective negative electrodes. Asection of each negative electrode was observed with an FIB-SIM. By theFIB-SIM observation, it is meant that the negative electrode isprocessed with a focused ion beam (FIB) so that its section is exposedto an outside and then the exposed section is observed with a scanningion microscope (SIM).

FIGS. 1 and 2 are SIM images of the negative electrode of the batteryA1, respectively. FIG. 2 is an enlarged representation of FIG. 1. FIGS.3 and 4 are SIM images of the negative electrode of the battery B1,respectively. FIG. 4 is an enlarged representation of FIG. 3. Since thenegative electrode is observed from above at an angle of 45 degreesrelative to its section, the actual thickness dimension of an object inFigures is given by multiplying a dimension measured using a scale ineach Figure (10 μm in FIGS. 1 and 3, and 1 μm in FIGS. 2 and 4) by theroot of 2. Accordingly, a thickness of the mix layer in the negativeelectrode (shown in FIG. 1) of the battery A1 is found to be about 25μm. Also, a thickness of the mix layer in the negative electrode (shownin FIG. 3) of the battery B1 is found to be about 42 μm.

In the mix layer shown in FIG. 1, dark portions indicate porous portionsof active material particle and white portions indicate nonporousportions of active material particle. As can be seen from FIG. 1, onlysurface portions of the active material particle are rendered porous inthe battery A1.

On the other hand, the increased dark portions and reduced whiteportions are observed in the negative electrode of the battery B1. Thisdemonstrates that the increased portions are rendered porous in thenegative electrode of the battery B1.

Also, the thickness increase of the mix layer in the battery B1 islarger than in the battery A1, as described above. This demonstratesthat a larger increase in porosity of the active material particlesresults in the larger thickness increase of the mix layer in thenegative electrode of the battery B1.

As can be appreciated from the forgoing, the use of an electrolytesolution containing dissolved carbon dioxide, in accordance with thepresent invention, retards increase in porosity of the active materialparticles and accordingly suppresses increase in thickness of theelectrode. In accordance with the present invention, a thicknessincrease of a battery after charge-discharge cycling can be suppressed,so that the battery has a high volumetric energy density.

A detailed reason why the use of a nonaqueous electrolyte containingdissolved carbon dioxide retards porosity increase of the activematerial particles is not clear. However, it is believed that the carbondioxide dissolved in the nonaqueous electrolyte acts to form, on asurface of the active material particle, a superior lithium-ionconducting film which restrains irreversible modification of the activematerial particle during charge and discharge and thus retard porosityincrease of the active material particle.

EXPERIMENT 3

In this Experiment, the effect of the mean particle diameter of thesilicon powder on the cycle characteristics was studied.

The procedure of Experiment 1 was followed, except that a silicon powderhaving a mean particle diameter of 20 μm was used, to fabricate abattery A2. Also, the procedure of Experiment 2 was followed, exceptthat a silicon powder having a mean particle diameter of 20 μm was used,to fabricate a battery B2.

In accordance with the procedure in Experiment 2, these batteries wereevaluated for cycle performance characteristics. The cycle life of eachbattery is indicated by an index when that of the battery A1 is taken as100. In Table 3, the cycle lives of the batteries A1 and B1 are alsoshown.

TABLE 3 Amount of Carbon Mean Particle Dioxide Dissolved in Diameter ofSilicon Electrolyte Solution Powder Battery (wt. %) (μm) Cycle Life A10.37 3 100 A2 0.37 20 64 B1 0 3 48 B2 0 20 22

As can be clearly seen from Table 3, the battery A1 using a siliconpowder having a mean particle diameter of not exceeding 10 μm as theactive material exhibits superior cycle performance, compared to thebattery A2. It has been therefore found that the charge-discharge cycleperformance improving effect obtained by using a nonaqueous electrolytecontaining dissolved carbon dioxide becomes significant when an activematerial powder having a mean particle diameter of not exceeding 10 μmis used.

EXPERIMENT 4

In this Experiment, the effect of the arithmetic mean surface roughnessRa of the current collector on the cycle characteristics was studied.

The procedure of Experiment 1 was followed, except that the currentcollector a1 was replaced by an electrolytic copper foil having anarithmetic mean surface roughness of 0.2 μm or 0.17 μm, to fabricate abatteries A3 and A4.

These batteries were evaluated for cycle performance characteristics inthe same manner as described above. The cycle life of each battery isindicated by an index when that of the battery A1 is taken as 100. InTable 4, the cycle life of the battery A1 is also shown.

TABLE 4 Arithmetic Mean Surface Roughness of Current Collector Battery(μm) Cycle Life A1 0.5 100 A3 0.2 80 A4 0.17 63

As can be clearly seen from Table 4, the batteries A1 and A3 using thecurrent collectors each having an arithmetic mean roughness Ra of 0.2 μmor above show superior cycle performance characteristics compared to thebattery A4 using the current collector having an arithmetic meanroughness Ra of below 0.2 μm. This is probably because the use of thecurrent collector having an arithmetic mean roughness Ra of 0.2 μm orabove increases a contact area of the current collector with the activematerial particles to result in the effective sintering that improvesadhesion between them, and also heightens the anchor effect of thebinder on the current collector to thereby further improve adhesionbetween the mix layer and the current collector and, as a result,improve a current-collecting capability of the electrode.

EXPERIMENT 5

In this Experiment, the effect of the sintering condition on the cyclecharacteristics was studied.

The procedure of Experiment 1 was followed, except that the heattreatment for sintering was performed at 600° C. for 10 hours, tofabricate a battery A5.

This battery was evaluated for cycle performance characteristics in thesame manner as described above. The cycle life of the battery isindicated by an index when that of the battery A1 is taken as 100. InTable 5, the cycle life of the battery A1 is also shown.

TABLE 5 Heat-Treating Condisions Battery for Negative Electrode CycleLife A1 400° C., 30 hrs. 100 A5 600° C., 10 hrs. 59

As can be clearly seen from Table 5, the battery A5 having the electrodemade through heat treatment at 600° C. for 10 hours shows markedlydeteriorated cycle performance characteristics, compared to the batteryA1 having the electrode made through heat treatment at 400° C. for 30hours. This is probably because when the heat treatment is carried outat 600° C., the adhesion between components within the electrode ismarkedly reduced, due to the decomposition of the binder serving todevelop adhesion, to result in the reduction of the current-collectingcapability.

EXPERIMENT 6

In this Experiment, the effect of the conductive powder loaded in themix layer on the cycle characteristics was studied.

The procedure of Experiment 1 was followed, except that a nickel powderhaving a mean particle diameter of 3 μm was loaded in the mix layer inthe amount of 20% of the total weight of the nickel powder and siliconpowder, to fabricate a battery A6.

This battery was evaluated for cycle performance characteristics in thesame manner as described above. The cycle life of the battery isindicated by an index when that of the battery A1 is taken as 100. InTable 6, the cycle life of the battery A1 is also shown.

TABLE 6 Battery Conductive Powder Cycle Life A1 None 100 A6 Ni 102

As can be clearly seen from Table 6, the battery A6 having a nickelpowder loaded in the mix layer shows improved cycle performancecharacteristics, compared to the battery A1 without loading of a nickelpowder in the mix layer. This is probably because the conductive powdersurrounds the active material particles to form a conductive networkthat improves a current collecting capability of the mix layer.

In the preceding embodiments, the mix layer was described to overlie onesurface of the current collector of the negative electrode. However, themix layer may be provided on each surface of the current collector. Insuch an instance, each current collector surface preferably has anirregular profile according to the present invention.

(TOF-SIMS Observation)

The present applicant has discovered that the negative electrodeobtained by sputter depositing an amorphous silicon thin film on aconductive metal foil current collector also shows a porosity increaseof active material during charge-discharge cycling, and that thisporosity increase can be retarded by using a nonaqueous electrolytecontaining carbon dioxide dissolved therein. The negative electrodehaving such a silicon thin film was utilized to fabricate batteries X1,Y1 and Y2. The battery X1 used a nonaqueous electrolyte containingdissolved carbon dioxide. The battery Y1 used a nonaqueous electrolyteto which carbon dioxide was not added. The battery Y2 used a nonaqueouselectrolyte to which carbon dioxide was not added but vinylene carbonate(VC) was added in the amount of 20% by weight. For the initially-chargedbatteries X1, Y1 and Y2, a surface of each negative electrode wasanalyzed by TOF-SIMS (time of flight-secondary ion mass spectrometry).

FIG. 5 is a positive ion TOF-SIMS spectrum and FIG. 6 is a negative ionTOF-SIMS spectrum. In FIGS. 5 and 6, “LiPF6+CO2” shows a spectrum forthe battery X1 of the present invention, “LiPF6” shows a spectrum forthe battery Y1 and “LiPF6+VC20 wt %” shows a spectrum for the batteryY2.

As can be clearly seen from FIGS. 5 and 6, the markedly reduced Si ionand Si-containing ions and the increased Li₂F⁺ ion at the surface of thenegative electrode, relative to the batteries Y1 and Y2, are detectedfor the battery X1. This demonstrates that use of the nonaqueouselectrolyte containing dissolved carbon dioxide results in the markedreduction in concentration of Si at the thin film surface. This is mostprobably due to the formation of an Si-free film on a surface of thesilicon active material. It is believed that this film is a stable filmhaving a high lithium-ion conducting capability and the formation ofsuch a film on a silicon surface suppresses property change of siliconand retards porosity increase of the silicon particles in acharge-discharge process during which lithium ions are stored andreleased from silicon.

On the other hand, it appears that a film containing an Si activematerial is formed in the negative electrode for the batteries Y1 andY2. The formation of such a film may be a probable cause of porosityincrease at the surface of the active material. The use of thenonaqueous electrolyte containing dissolved carbon dioxide is consideredto prevent formation of such a film and successfully retard increase inporosity of the active material.

Also in the present invention, such a stable film having a highlithium-ion conducting capability is believed to deposit on a surface ofthe active material particle, as similar to the above, and suppresschange in property of the active material and accordingly retardporosity increase of the active material particle in a charge-dischargeprocess during which lithium ions are stored and released from theactive material particle.

EXPERIMENT 7

In this Experiment, the effect of the amount of carbon dioxide dissolvedin the electrolyte solution on the cycle characteristics was studied.

(Preparation of Positive and Negative Electrodes)

The procedure of Experiment 1 was followed to prepare positive andnegative electrodes.

(Preparation of Electrolyte Solution)

In accordance with the procedure of Experiment 1, the electrolytesolution x was prepared and then a carbon dioxide gas was blown into theelectrolyte solution x to prepare the electrolyte solution a1.

The electrolyte solutions x and a1 at the volume ratio specified inTable 7 were mixed under an argon gas atmosphere to prepare electrolytesolutions a2, a3 and a4.

TABLE 7 Amount of Carbon Content of Content of Dioxide Dissolved inElectrolyte Electrolyte Electrolyte Solution Electrolyte Solution XSolution a1 (wt. %) Solution (vol. %) (vol. %) 0.37 a1  0 100  0.185 a250 50 0.0925 a3 75 25 0.037 a4 90 10 0 x 100   0

(Fabrication of Batteries)

The procedure of Experiment 1 was followed to fabricate the rechargeablelithium battery A1 using the electrolyte solution a1.

In addition, the procedure of Experiment 1 was followed but under anambient-pressure argon gas atmosphere to fabricate a battery A7 usingthe electrolyte solution a2, a battery A8 using the electrolyte solutiona3, a battery A9 using the electrolyte solution a4 and a battery B3using the electrolyte solution x.

(Evaluation of Charge-Discharge Cycle Characteristics)

The above-fabricated batteries were evaluated for cycle performancecharacteristics in the same manner as described above. The evaluationresults are shown in Table 8.

The cycle life of each battery is indicated by an index when that of thebattery A1 is taken as 100. In Table 8, the cycle life of the battery A1is also shown.

Also, FIG. 9 shows the relationship between the amount of carbon dioxidedissolved in the electrolyte solution and the cycle life of eachbattery.

TABLE 8 Amount of Carbon Dioxide Dissolved in Electrolyte SolutionElectrolyte (wt. %) Battery Solution Cycle Life 0.37 A1 a1 100  0.185 A7a2 98 0.0925 A8 a3 95 0.037 A9 a4 71 0 B3 x 38

As can be clearly seen from Table 8 and FIG. 9, the batteries A1 andA7-A9 using the electrolyte solution each containing at least 0.01% byweight of dissolved carbon dioxide exhibit longer cycle lives, comparedto the battery B3 using the electrolyte solution x in which carbondioxide was not dissolved. Also, if the amount of carbon dioxidedissolved in the electrolyte solution is at least 0.05% by weight, thecycle life exceeds approximately 80% of its saturation value. Further,as the amount of carbon dioxide dissolved in the electrolyte solutionincreases to 0.1% by weight, the cycle life approaches its saturationvalue.

The forgoing demonstrates that the amount of carbon dioxide dissolved inthe electrolyte solution is preferably at least 0.01% by weight, morepreferably at least 0.05% by weight, further preferably at least 0.1% byweight.

EXPERIMENT 8

In this Experiment, the effect of fluorine introduced in the nonaqueouselectrolyte on the cycle characteristics was studied.

(Preparation of Electrolyte Solution)

1 mole/liter of LiPF₆ was dissolved in a mixed solvent containingethylene carbonate and diethyl carbonate at a 3:7 ratio by volume toprepare an electrolyte solution P0. This electrolyte solution P0 wascooled to 5° C. Thereafter, a carbon dioxide gas was blown at a flowrate of 300 ml/min into the electrolyte solution P0 under a carbondioxide atmosphere. Blowing was continued (for about 30 minutes) until aweight of the electrolyte solution was leveled off. The resultant waselevated in temperature to 25° C. to prepare an electrolyte solution P1.

The weight of the electrolyte solution subsequent to blowing of a carbondioxide gas was measured under a carbon dioxide gas atmosphere to find achange in weight of the electrolyte solution. From the finding, theamount of a carbon dioxide gas dissolved in the electrolyte solution was0.37% by weight.

The procedure used to prepare the electrolyte solution P0 was followed,except that LiPF₆ was replaced by LiBF₄, to prepare an electrolytesolution B0. As analogous to the electrolyte solution P1, a carbondioxide gas was blown into the electrolyte solution B0 to prepare anelectrolyte solution B1.

The procedure used to prepare the electrolyte solution P0 was followed,except that LiPF₆ was replaced by LiN(C₂F₅SO₂)₂, to prepare anelectrolyte solution N0. As analogous to the electrolyte solution P1, acarbon dioxide gas was blown into the electrolyte solution N0 to preparean electrolyte solution N1.

The procedure used to prepare the electrolyte solution P0 was followed,except that LiPF₆ was replaced by LiClO₄, to prepare an electrolytesolution C0. As analogous to the electrolyte solution P1, a carbondioxide gas was blown into the electrolyte solution C0 to prepare anelectrolyte solution C1.

(Fabrication of Battery)

The same positive and negative electrodes as used in Experiment 1 andthe electrolyte solutions P1, B1, N1 and C1 were used to fabricaterechargeable lithium batteries AP1, AB1, AN1 and AC1. Batteryfabrication was performed under a normal temperature and pressure carbondioxide gas atmosphere.

Also, the same positive and negative electrodes as in Experiment 1 andthe electrolyte solutions P0, B0, N0 and C0 were used to fabricaterechargeable lithium batteries AP0, AB0, AN0 and AC0. Batteryfabrication was performed under a normal temperature and pressure argongas atmosphere.

(Evaluation of Charge-Discharge Cycle Characteristics)

The above-fabricated batteries AP1, AB1, AC1, AP0, AB0 and AC0 wereevaluated for charge-discharge cycle performance characteristics. Eachbattery at 25° C. was charged at a constant current of 14 mA to 4.2 V,charged at a constant voltage of 4.2 V to a current of 0.7 mA and thendischarged at a current of 14 mA to 2.75 V. This was recorded as a unitcycle of charge and discharge.

The batteries AN1 and AN0 were charged and discharged by following theabove charge-discharge sequence except that the constant-currentcharging was continued to 4.0 V.

The battery was cycled to determine the number of cycles after which itsdischarge capacity fell down to 80% of its first-cycle dischargecapacity and the determined cycle number was recorded as a cycle life.The results are shown in Table 9.

In Table 9, the cycle life A is indicated by an index when a cycle lifeof the battery AP1 is taken as 100. The cycle life B is indicated by anindex when a cycle life of the battery using the electrolyte solutioncontaining dissolved carbon dioxide is taken as 100.

TABLE 9 Dissolved Electrolyte Carbon Cycle Life Cycle Life Lithium SaltSolution Battery Dioxide A B LiPF₆ P1 AP1 Present 100 100 LiPF₆ P0 AP0Absent 38 38 LiBF₄ B1 AB1 Present 44 100 LiBF₄ B0 AB0 Absent 26 59LiN(C₂F₅SO₂)₂ N1 AN1 Present 87 100 LiN(C₂F₅SO₂)₂ N0 AN0 Absent 39 45LiClO₄ C1 AC1 Present 42 100 LiClO₄ C0 AC0 Absent 39 93

As can be clearly seen from Table 9, the batteries AP1, AB1, AN1 and AC1each using the electrolyte solution containing dissolved carbon dioxideexhibit longer cycle lives than the batteries AP0, AB0, AN0 and AC0 inwhich carbon dioxide was not dissolved. Particularly, the batteries AP1,AB1 and AN1 using a fluorine-containing lithium salt show larger cyclelife improvements, compared to the battery AC1 using a fluorine-freelithium salt. This appears to demonstrate that inclusion of thefluorine-containing lithium salt either promotes the action of carbonoxide to form a superior film or further improves a property of a filmformed by the action of carbon dioxide. The fluorine-containing lithiumsalt may be decomposed during charge and discharge to produce a hydrogenfluoride or the like which affects the action of carbon dioxide to forma superior film. Such a film may reduce the amount of lithium ions thatare consumed to form films on new surfaces produced as a result ofdivision of active material during a charge-discharge reaction, therebysuppressing a decrease of a charge-discharge efficiency. Also, since thefilm formed on a surface of the active material particle has a superiorlithium-ion conducting capability, it may allow a charge-dischargereaction to occur in more uniformly distributed regions of the activematerial particle. This is believed to lessen a strain produced when theactive material undergoes a biased volumetric change as it stores andreleases lithium and, as a result, improve a charge-dischargeefficiency.

In the preceding embodiments, the negative current collector wasdescribed to have one irregular surface on which the active materiallayer was placed. The present invention is not limited to such anarrangement. The current collector may have irregularities on its bothsurfaces. In such an case, the active material layer may be placed oneach irregular surface of the current collector to constitute a negativeelectrode.

EXPERIMENT 9

(Preparation of Electrolyte Solution)

1 mole/liter of LiPF₆ was dissolved in a mixed solvent containingethylene carbonate (EC) and diethyl carbonate (DEC) at a 3:7 ratio byvolume to prepare an electrolyte solution ED0.

This electrolyte solution ED0 was cooled to 5° C. Thereafter, a carbondioxide gas was blown at a flow rate of 300 ml/min into the electrolytesolution ED0 under a carbon dioxide atmosphere. Blowing was continued(for about 30 minutes) until a weight change of the electrolyte solutionwas leveled off. The resultant was elevated in temperature to 25° C. toprepare an electrolyte solution ED1. The weight of the electrolytesolution subsequent to blowing of a carbon dioxide gas was measuredunder a carbon dioxide gas atmosphere to find a change in weight of theelectrolyte solution prior to and subsequent to blowing of a carbondioxide gas. From the finding, the amount of a carbon dioxide gasdissolved in the electrolyte solution was calculated at 0.37% by weight.

The procedure used to prepare the electrolyte solution ED1 was followed,except that propylene carbonate (PC) was used as the cyclic carbonateand diethyl carbonate (DEC) was used as the chain carbonate, to preparean electrolyte solution. As analogous to the electrolyte solution ED1, acarbon dioxide gas was dissolved in this electrolyte solution to preparean electrolyte solution PD1.

The procedure used to prepare the electrolyte solution ED1 was followed,except that propylene carbonate (PC) was used as the cyclic carbonateand methyl ethyl carbonate (MEC) was used as the chain carbonate, toprepare an electrolyte solution. As analogous to the electrolytesolution ED1, a carbon dioxide gas was dissolved in this electrolytesolution to prepare an electrolyte solution PM1.

The procedure used to prepare the electrolyte solution ED1 was followed,except that ethylene carbonate (EC) was used as the cyclic carbonate andmethyl ethyl carbonate (MEC) was used as the chain carbonate, to preparean electrolyte solution. As analogous to the electrolyte solution ED1, acarbon dioxide gas was dissolved in this electrolyte solution to preparean electrolyte solution EM1.

Ethylene carbonate (EC) as the cyclic carbonate and diethyl carbonate(DEC) as the chain carbonate were mixed at a 1:1 ratio by volume toprepare a mixed solvent. As analogous to the electrolyte solution ED1,LiPF₆ and carbon dioxide were dissolved in the mixed solvent to preparean electrolyte solution EDM1.

The amounts of carbon dioxide dissolved in the electrolyte solutionsPD1, PM1, EM1 and EDM1 were 0.36% by weight, 0.64% by weight, 0.54% byweight and 0.46% by weight, respectively.

(Fabrication of Battery)

The same positive and negative electrodes as used in Experiment 1 andthe above-prepared electrolyte solutions ED0, ED1, PD1, PM1, EM1 andEDM1 were used. Otherwise, the procedure of Experiment was followed tofabricate rechargeable lithium batteries. During fabrication of thebatteries using the electrolyte solutions ED1, PD1, PM1, EM1 and EDM1,each set of the positive electrode, negative electrode and electrolytesolution was inserted into an aluminum laminate outer casing under anormal temperature and pressure carbon dioxide gas atmosphere.

In the fabrication of the battery using the electrolyte solution ED0 towhich carbon dioxide was not dissolved, a set of the positive electrode,negative electrode and electrolyte solution was inserted into analuminum laminate outer casing under a normal temperature and pressureargon gas atmosphere.

The batteries using the electrolyte solutions ED0, ED1, PD1, PM1, EM1and EDML were designated as AED0, AED1, APD1, APM1, AEM1 and AEDM1,respectively.

The type of the electrolyte solution and the presence of dissolvedcarbon dioxide therein, for the above-fabricated batteries, are listedin the following Table.

TABLE 10 Electrolyte Electrolyte Dissolved Carbon Solution SolutionBattery Dioxide LiPF₆/EC + DEC ED0 AED0 Absent (3/7) LiPF₆/EC + DEC ED1AED1 Present (3/7) LiPF₆/PC + DEC PD1 APD1 Present (3/7) LiPF₆/PC + MECPM1 APM1 Present (3/7) LiPF₆/EC + MEC EM1 AEM1 Present (3/7) LiPF₆/EC +DMC EDM1 AEDM1 Present (1/1)

(Evaluation of Charge-Discharge Cycle Characteristics)

The above-fabricated batteries were each evaluated for charge-dischargecycle performance characteristics in the same manner as in Experiment 1.The cycle life of each battery is indicated therein by an index whenthat of the battery AED1 is taken as 100. The results are shown in Table11.

TABLE 11 Amount of Dissolved Carbon Dioxide in Electrolyte ElectrolyteElectrolyte Solution Cycle Solution Solution Battery (wt. %) LifeLiPF₆/EC + DEC ED0 AED0 0 38 (3/7) LiPF₆/EC + DEC ED1 AED1 0.37 100(3/7) LiPF₆/PC + DEC PD1 APD1 0.36 108 (3/7) LiPF₆/PC + MEC PM1 APM10.64 75 (3/7) LiPF₆/EC + MEC EM1 AEM1 0.54 91 (3/7) LiPF₆/EC + DMC EDM1AEDM1 0.46 70 (1/1)

As can be clearly seen from the results shown in Table 11, the batteryAPD1 using propylene carbonate as the cyclic carbonate and diethylcarbonate as the chain carbonate shows superior cycle performancecharacteristics, compared to the battery AED1 using ethylene carbonateas the cyclic carbonate and diethyl carbonate as the chain carbonate. Inthe case of a graphite negative electrode, propylene carbonate is notgenerally used. However, in the case of a silicon negative electrode,the use of propylene carbonate has been found to provide a good result.This is probably because when propylene carbonate is used, theelectrolyte solution increases its viscosity and becomes easier toimpregnate into the electrode than when ethylene carbonate is used and,as a result, a more homogeneous film is formed on a silicon surface tosuppress a capacity drop at an initial stage of cycling.

As can be appreciated, the cycle life improving effect has been alsoobtained for the other batteries APM1, AEM1 and AEDM1 when containingdissolved carbon dioxide in their respective electrolyte solutions.

REFERENCE EXPERIMENT

(Preparation of Carbon Negative Electrode)

Artificial graphite as a negative active material and astyrene-butadiene rubber as a binder were mixed in an aqueous solutionof carboxy methyl cellulose as a thickener so that the mixture containedthe active material, binder and thickener in the ratio by weight of95:3:2. The mixture was then kneaded to prepare a negative electrodeslurry. The prepared slurry was applied onto a copper foil as a currentcollector, dried and rolled by a pressure roll. Subsequent attachment ofa current collecting tab resulted in the preparation of a negativeelectrode.

(Preparation of Positive Electrode)

90 parts by weight of LiCoO₂ powder and 5 parts by weight of artificialgraphite powder as an electric conductor were mixed in a 5 wt. %N-methylpyrrolidone aqueous solution containing 5 parts by weight ofpolytetrafluoroethylene as a binder to provide a cathode mix slurry.This slurry was coated by a doctor blade process on an aluminum foil asa positive current collector and then dried to form a layer of positiveactive material. A positive tab was attached onto an aluminum foilregion left uncoated with the positive active material layer to preparea positive electrode.

(Preparation of Nonaqueous Electrolyte)

1 mole/liter of LiPF₆ was dissolved in a mixed solvent containingethylene carbonate and diethyl carbonate at a 3:7 ratio by volume toprepare a solution.

Vinylene carbonate was added to this solution in the amount of 2% byweight to prepare a nonaqueous electrolyte c2.

Carbon dioxide was blown for 30 minutes into the nonaqueous electrolytec2 at 25° C. until carbon dioxide was dissolved therein to saturation.As a result, a nonaqueous electrolyte c1 was obtained. The amount ofdissolved carbon dioxide was 0.37 weight %.

The nonaqueous electrolytes c1 and c2 are clarified as follows:

-   -   nonaqueous electrolyte c1: a nonaqueous electrolyte in which CO₂        was dissolved;    -   nonaqueous electrolyte c2: a nonaqueous electrolyte in which CO₂        was not dissolved.

(Fabrication of Batteries)

Rechargeable lithium batteries were fabricated using the above-preparednegative electrode, positive electrode and nonaqueous electrolytes. Thepositive and negative electrodes were rolled up in a cylindricalconfiguration with a porous polyethylene separator between them. Thiselectrode group and each nonaqueous electrolyte were inserted in anouter casing made of an aluminum laminate. The outer casing was heatsealed at its peripheries such that leading ends of the positive andnegative current collecting tabs extended outwardly from the outercasing, thereby completing fabrication of the battery.

The particulars of the fabricated batteries are listed in Table 12.

TABLE 12 Thickness (mm) 3.6 Width (mm) 35 Height (mm) 62 DesignedCapacity (mAh) 600 Number of Turns 9 Thickness of Negative Active 53.5Material Layer (μm)

The battery fabricated using the nonaqueous electrolyte c1 wasdesignated as C1. The battery fabricated using the nonaqueouselectrolyte c2 was designated as C2. Fabrication of the battery C1 wascarried out in the high-purity carbon dioxide gas atmosphere.

(Charge-Discharge Cycle Test)

The thus-fabricated rechargeable lithium batteries C1 and C2 weresubjected to a charge-discharge cycle test. Each battery at 25° C. wascharged at a constant current of 600 mA to 4.2 V, charged at a constantvoltage of 4.2 V to 30 mA and then discharged at a current of 600 mA to2.75 V. This was recorded as a unit cycle of charge and discharge. The500th-cycle discharge capacity was divided by the 1st-cycle dischargecapacity to give a capacity retention rate as shown in Table 13. Table13 also shows a thickness increase of the battery after 500 cycles and athickness increase of the active material per layer of the electrode ascalculated from the value for the thickness increase of the battery.

TABLE 13 Thickness Thickness Increase of Increase of Active CapacityRetention Battery After Material Layer Per After 500 Cycles 500 CyclesLayer of Electrode Battery (%) (μm) (μm) C1 88.9 148 8 C2 88.2 150 8

As can be clearly seen from the results shown in Table 13, dissolving ofcarbon dioxide in a nonaqueous electrolyte is little effective to retarddeterioration of cyclic performance and suppress increase in thicknessof the battery using a carbon material as the negative active material.

1. A rechargeable lithium battery including a negative electrode made bysintering a layer of a mixture of silicon particles, which undergo aporosity increase that advances inside from particle surfaces duringcharge and discharge, and a binder on a surface of a conductive metalfoil current collector, a positive electrode and a nonaqueouselectrolyte, characterized in that said nonaqueous electrolyte containscarbon dioxide dissolved therein in addition to carbon dioxide formedduring fabrication of the battery and forms a film having a lithium-ionconducting capability on a surface of said negative electrode, whereinthe amount of added carbon dioxide dissolved in said nonaqueouselectrolyte is at least 0.1 by weight.
 2. The rechargeable lithiumbattery as recited in claim 1, characterized in that said sintering isperformed under a non-oxidizing atmosphere.
 3. The rechargeable lithiumbattery as recited in claim 1, characterized in that carbon dioxide isfurther contained in an inner space of the battery.
 4. The rechargeablelithium battery as recited in claim 1, characterized in that saidnonaqueous electrolyte contains a cyclic carbonate.
 5. The rechargeablelithium battery as recited in claim 1, characterized in that saidnonaqueous electrolyte contains a mixed solvent of a cyclic carbonateand a chain carbonate.
 6. The rechargeable lithium battery as recited inclaim 4, characterized in that said cyclic carbonate includes ethylenecarbonate and/or propylene carbonate.
 7. The rechargeable lithiumbattery as recited in claim 4, characterized in that said cycliccarbonate is ethylene carbonate.
 8. The rechargeable lithium battery asrecited in claim 4, characterized in that said cyclic carbonate ispropylene carbonate.
 9. The rechargeable lithium battery as recited inclaim 5, characterized in that said chain carbonate includes at leastone of dimethyl carbonate, diethyl carbonate and methyl ethyl carbonate.10. The rechargeable lithium battery as recited in claim 1,characterized in that said nonaqueous electrolyte further contains afluorine-containing compound.
 11. The rechargeable lithium battery asrecited in claim 10, characterized in that said fluorine-containingcompound is a fluorine-containing lithium salt.
 12. The rechargeablelithium battery as recited in claim 11, characterized in that saidfluorine-containing lithium salt is LiXF_(y) (wherein X is P, As, Sb, B,Bi, Al, Ga or In; y is 6 if X is P, As or Sb and y is 4 if X is B, Bi,Al, Ga or In) or LiN(C_(m)F_(2m+1)SO₂)(C_(n)F_(2n+1)SO₂) (wherein m andn are independently integers of 1-4).
 13. The rechargeable lithiumbattery as recited in claim 11, characterized in that saidfluorine-containing lithium salt is at least one selected from LiPF₆,LiBF₄ and LiN(C₂F₅SO₂)₂.
 14. The rechargeable lithium battery as recitedin claim 1, characterized in that said silicon particles have a meanparticle diameter of 10 μm or below.
 15. The rechargeable lithiumbattery as recited in claim 1, characterized in that said currentcollector has an arithmetic mean surface roughness Ra of at least 0.2μm.
 16. The rechargeable lithium battery as recited in claim 1,characterized in that said current collector comprises a copper foil, acopper alloy foil or a metal foil having a copper or copper alloysurface layer.
 17. The rechargeable lithium battery as recited in claim1, characterized in that said current collector comprises anelectrolytic copper foil, an electrolytic copper alloy foil or a metalfoil having an electrolytic copper or copper alloy surface layer. 18.The rechargeable lithium battery as recited in claim 1, characterized inthat said binder remains even after a heat treatment for the sintering.19. The rechargeable lithium battery as recited in claim 1,characterized in that said binder comprises polyimide.
 20. Therechargeable lithium battery as recited in claim 1, characterized inthat an electric conductor is mixed in said mixture layer.
 21. A methodfor fabricating a rechargeable lithium battery including a negativeelectrode, a positive electrode and a nonaqueous electrolyte,characterized as comprising the steps of: providing a layer of a mixtureof silicon particles, which particles undergo a porosity increase thatadvances inside from particle surfaces during charge and discharge, anda binder on a surface of a conductive metal foil as a current collectorand sintering the mixture layer while being placed on said surface ofthe conductive metal foil to prepare said negative electrode; dissolvingcarbon dioxide in said nonaqueous electrolyte in an amount of at least0.1% by weight; and assembling a rechargeable lithium battery using saidnegative electrode, positive electrode and nonaqueous electrolyte,wherein a film having a lithium-ion conducting capability is formed on asurface of said negative electrode by the dissolving of the carbondioxide in the said nonaqueous electrolyte.
 22. The method forfabricating a rechargeable lithium battery as recited in claim 21,characterized in that said sintering is performed under a non-oxidizingatmosphere.
 23. The method for fabricating a rechargeable lithiumbattery as recited in claim 21, characterized in that the step ofdissolving carbon dioxide in the nonaqueous electrolyte includes a stepof blowing gaseous carbon dioxide into the nonaqueous electrolyte. 24.The method for fabricating a rechargeable lithium battery as recited inclaim 21, characterized in that the step of assembling a rechargeablelithium battery includes a step of assembling a rechargeable lithiumbattery under the atmosphere including carbon dioxide.